In the art of microwave cooking, it is desirable that the exterior of certain types of food be browned or made crisp to have the texture and appearance of conventionally fried, baked, or roasted foods. The recooking of foods in a conventional oven after microwave cooking to achieve this condition is complicated and inconvenient for the user. Therefore, attempts have been made to develop food receptacles and packaging which can obtain the desired browning or crisping effect.
A recent development has been the use of microwave susceptors for food packaging and receptacles. For example, as disclosed in U.S. Pat. No. 4,641,005 to Seiferth, a susceptor structure is made of a plastic film coated on one side with a metal layer and bonded to a backing sheet of paper, paperboard, or non-thermoplastic material. The metal layer is designed to absorb microwave radiation and generate conductive heat to be transmitted through the plastic film to the surface of the food. However, such metallized film susceptors have the problem that the density of the metal layer necessary to provide sufficient heat to cook the food surface can generate excessive heat in other areas of the sheet which are not in contact with the food. Excessive heat can cause cracking, crazing, shrinking, or embrittlement of the plastic film. Susceptor temperatures above 450 degrees may result in combustion or volatilization of the various susceptor components, and subsequent contamination of the food.
One attempt to correct the above-mentioned problem of areas of excessive heat in susceptors is shown in U.S. Pat. No. 4,735,513 to Watkins, wherein the metal layer is formed as one or more separate islands of smaller area than the food receptacle as a whole, with a shape or shapes corresponding to that of the food to be cooked. This allows heat to be generated only in the defined areas intended to be placed in contact with food. However, the individual metal layers have to be separately positioned and sandwiched in between protective backing layers in one fabrication step, in order to properly position them at the intended sites of the food, to provide dimensional stability, and to prevent them from becoming delaminated from the plastic film. This requirement results in a more complicated and exacting fabrication process, and requires the fabrication of many different types of susceptor configurations depending on the shapes of food to be cooked or the food receptacles to be made from the susceptor material.
In view of the above-mentioned problems and disadvantages of the prior art, it is a principal object of the invention to provide an improved susceptor structure which efficiently converts microwave radiation into heat and distributes the heat relatively evenly over its surface area so that excessive heat generation, deterioration of the susceptor, and burning or contamination of the food are prevented. It is a further object of the invention that the improved susceptor structure can be easily fabricated in web or sheet form without requiring the complicated positioning of constituent parts or many different shapes for different uses.